1. Field of the Invention
This invention relates to an active matrix type liquid crystal display device in which thin film transistors are used as non-linear switching elements, and more particularly to an active matrix type liquid crystal display device in which additional capacitors are formed.
2. Description of the Prior Art
In an active matrix type liquid crystal display device, thin film transistors (TFTs) are used as switching elements for driving picture elements. It is often necessary to use an additional capacitor for each picture element in order to improve the display quality.
FIG. 3 shows a substrate of a prior art active matrix type liquid crystal display device. TFTs and additional capacitors are formed on the substrate. A plurality of parallel gate bus lines 2 are formed in horizontally, and a plurality of source bus lines 3 are formed vertically on an insulating substrate 7 (FIG. 4). In the vicinity of each intersection of the gate bus lines 2 and source bus lines 3, a TFT 5 is formed. Each TFT 5 drives a picture element electrode 4 which is disposed in each region surrounded by two adjacent gate bus lines 2 and source bus lines 3. Under the picture element electrode 4, a capacitor electrode 6 is formed. Between the substrate assembly of FIG. 4 and an opposing substrate assembly (not shown), a liquid crystal material is sealed to construct a display device. Each picture element electrode 4 constitutes a picture element.
The structure of the substrate assembly will be described in more detail with reference to FIG. 4 which is a sectional view taken along line IV--IV of FIG. 3. The gate electrode 8 of the TFT 5 and the capacitor electrode 6 are formed on the surface of the insulating substrate 7. The gate electrode 8 is one part of the gate bus line 2. The gate electrode 8 and capacitor electrode 6 can be formed simultaneously, and are made of the same material. An anodic oxidation film 9 is formed on each of the gate electrode 8 and capacitor electrode 6. A gate insulating layer 10 is formed on the anodic oxidation films 9 and the remaining surface of the insulating substrate 7.
The TFT 5 further comprises an i-amorphous silicon (a-Si) film 11, an insulating film 12, n.sup.+ -a-Si films 13, a source electrode 14a, and a drain electrode 14b which are laminated on the gate insulating layer 10. The source electrode 14a and drain electrode 14b are made of the same material as that of the source bus lines 3, and formed simultaneously with the source bus lines 3. The picture element electrode 4 is formed on the gate insulating layer 10, and electrically connected to the drain electrode 14b.
In the above-mentioned structure, a dielectric layer consisting of the anodic oxidation film 9 and the gate insulating layer 10 is formed between the picture element electrode 4 and the additional capacitor electrode 6, so that the picture element electrode 4 and the capacitor electrode 6 constitute an additional capacitor which is connected in parallel to the capacitor caused by the liquid crystal. This additional capacitor store charges when the TFT 5 is On to drive the picture element electrode 4, and substantially retains the voltage applied to the electrode 4 until the next frame scanning period.
In a prior art active matrix type liquid crystal display device having such a configuration, the picture element electrode 4 is electrically insulated from the capacitor electrode 6 by the dielectric layer consisting of the anodic oxidation film 9 and the gate insulating layer 10. However, pinholes often form in the dielectric layer due to the presence of a foreign body in the dielectric layer. In such a case, the picture element electrode 4 disposed on the dielectric layer falls into electrical continuity with the capacitor electrode 6 formed under the dielectric layer. As a result, when the TFT 5 connected to the picture element electrode 4 is ON, current leaks from the picture element electrode 4 to the capacitor electrode 6 to cause a display defect. In a prior art active matrix type liquid crystal display device, therefore, the existence of a pinhole in the two-layered structure between the picture element electrode 4 and the additional capacitor electrode 6 causes an inferior production yield.